Sunday, March 23, 2014

New paper finds Arctic sea ice was much less than present-day during the Holocene Climate Optimum ~6,000 years ago

A new paper published in Quaternary Science Reviews finds Arctic sea ice extent and thickness was much less than present-day conditions during the Holocene Climate Optimum from ~10,000-6,000 years ago. According to the authors, "Arctic Ocean sea ice proxies generally suggest a reduction in sea ice during parts of the early and middle Holocene (∼6000–10,000 years Before the Present) compared to present day conditions." The authors show 8 different proxy studies reveal extended periods lasting hundreds of years without perennial sea ice in the Arctic [ice-free conditions], and find solar insolation explains these changes.

Top graph shows simulated annual mean sea ice thickness [orange curve] was much less during the Holocene Climate Optimum ~13,000-6,000 years ago compared to the end of the 20th century at right side of graph. Bottom graph shows multiple proxies of sea ice with darker green indicating periods of less sea ice. Modern sea ice is at high levels in comparison to the rest of the Holocene. Fig. 4.

Annual mean sea ice thickness for the three different simulations (Panel a) compared with results from published paleo-sea ice studies (Panel b). Black curve: constant surface albedo; red curve: dynamic surface albedo parameterization. The simulation implemented with a dynamic surface albedo parameterization was run from present time and backwards to address the importance of the initial state of the sea ice cover. The annual mean sea ice thickness from this simulation (orange curve) reveals a hysteresis of ∼1000 years. The annual mean insolation at 80°N shown with a stippled curve is based on the algorithm presented by Berger (1978). To compare the results from different paleo-sea ice studies a scale of sea ice concentration was inferred using the approach by Jakobsson et al. (2010). This scale must be considered as highly qualitative because none of the paleo-sea ice proxies provide absolute measures of past sea ice concentrations.

Abstract

Arctic Ocean sea ice proxies generally suggest a reduction in sea ice during parts of the early and middle Holocene (∼6000–10,000 years Before the Present) compared to present day conditions. This sea ice minimum has been attributed to the northern hemisphere Early Holocene Insolation Maximum (EHIM) associated with Earth's orbital cycles. Here we investigate the transient effect of insolation variations during the final part of the last glaciation and the Holocene by means of continuous climate simulations with the coupled atmosphere–sea ice–ocean column model CCAM. We show that the increased insolation during EHIM has the potential to push the Arctic Ocean sea ice cover into a regime dominated by seasonal ice, i.e. ice free summers. The strong sea ice thickness response is caused by the positive sea ice albedo feedback. Studies of the GRIP ice cores and high latitude North Atlantic sediment cores show that the Bølling–Allerød period (c. 12,700–14,700 years BP) was a climatically unstable period in the northern high latitudes and we speculate that this instability may be linked to dual stability modes of the Arctic sea ice cover characterized by e.g. transitions between periods with and without perennial sea ice cover.

http://www.sciencedirect.com/science/article/pii/S0277379113004162?np=yThe present study points at a potentially large impact of the increased SW [short wave solar radiation] forcing during EHIM [Early Holocene Insolation Maximum, 9,000 to 6,000 years ago] through the surface albedo feedback, leading to a breakdown of the perennial sea ice cover into a state dominated by ice free summers. Above we have discussed some mechanisms and feedback processes that are not included in the present model study. Although some of the in this model omitted mechanisms are believed to be important (e.g. the surface albedo feedback associated with vegetation) the current understanding of the climate system as a whole is far from complete. Evidence of problems with the coupled GCMs [models] can be found in the PMIP [models of paleoclimate] literature. For instance Jiang et al. (2012) show that 35 out of 36 PMIP models produce colder than present day climate in China during mid-Holocene which is in stark contrast to available multiproxy data for the same time and region indicating 1–5 °C warmer than present day conditions. Suffice to say that further research on the coupled global climate system is needed before any conclusive results regarding the evolution of the Arctic sea ice cover during Holocene can be reached through climate modelling.

Several studies suggest that the GHG concentrations were lower during the [Early Holocene Insolation Maximum, ∼ 9,000 years ago] (Indermuhle, 1999, Brook et al., 2000 and Sowers et al., 2003). However, when running the model with GHG [greenhouse gas] concentrations estimated from paleo-proxy data (LeGrande and Schmidt, 2009) for 9000 years BP [before present] yields only a moderate effect on the ice cover thickness of typically ∼0.1 m. This is consistent with the results of CAPE Project members (2001) showing also only marginal effects of the GHG [greenhouse gas] concentration variations on the Arctic climate.